a is a function which would already be defined at this point. Anything after the exclamation point is arguments to the callback function. Any following code is

a is a function which would already be defined at this point. Anything after the exclamation point is arguments to the callback function. Any following code is wrapped in an anonymous function passed as the final argument to the function a. Anything on the left-hand side of the assignment (in this case, x and y) are arguments passed to this anonymous function. A concrete example of why you would do this would probably be useful.